1. Field of the Invention
The present invention relates generally to polymers with pendent 1H-azoles and more specifically to c-substituted, 1H-azoles for amphoteric, solvent-less proton conductivity.
2. Description of the Prior Art
The search for efficient/alternative fuel sources has opened many opportunities for the development of both known and new materials. Nafion, for example, is a polymeric material first synthesized in the late 1960's that is used as the proton exchange membrane (PEM) in hydrogen fuel cells of today. The chemical composition of Nafion shows a morphology consisting of both hydrophobic and hydrophilic sites where upon hydration forms channels in which movement of cations, specifically the hydrogen cation or proton, hop from one hydrophilic site to another but does not permit the passage of anions or electrons. This “hopping” mechanism is called the Grotthus mechanism.
Rapid proton transport in water by both structural and vehicular motion of protic defects in the hydrogen bonded network is well known. Many applications currently rely on the high conductivity of aqueous systems, with fuel cell membranes among them. However, other amphoteric molecules, those capable of acting simultaneously as a Brønsted acid (proton donor) and base (proton acceptor), have also been shown to exhibit proton transport by similar means. (A. Kawada et al., J. Chem. Phys., 52 (6), 3121-25 (1970)). Many recent materials designed for anhydrous proton transport belong to the azole family of amphoteric, five-membered heterocycles, and several interesting synthetic strategies have been developed to allow access to azoles that retain an intact NH proton. (J. C. Loren et al., Synlett, 18, 2847-50 (2005); Z. P. Demko et al., J. Org. Chem., 66, 7945-50 (2001); V. Aureggi et al., Angew. Chem. Int. Ed., 46, 8440-44 (2007); S. Martwiset et al., Solid State Ionics, 178, 1398-1403 (2007)). Among these materials, triazoles and imidazoles have been widely studied as plasticizers to polyacids and pendant moieties within polymer matrices.
Substituted tetrazoles are a unique class of heterocycles with a wide range of potential applications ranging from coordination to medicinal chemistry and many uses in material science. (P. Lin et al., Dalton Trans. 2388 (2005); A. K. Gupta et al., Synlett, 12, 2227 (2004); E. H. White et al., Tetrahedron Lett., 21, 758 (1961); H. Zue, Chem. Mater. 17, 19 (2005)). These compounds are usually obtained by the addition of azide salts to nitriles under heating conditions. (S. J. Wittenberger, J. Org. Prep. Proced. Intl. 26, 499 (1994); B. E. Huff et al., Tetrahedron Lett., 34, 8011 (1993); J. Sauer et al., Tetrahedron, 11, 241 (1960)). These procedures have been known since the 1930s, often employing toxic metals, expensive reagents and harsh reaction conditions. Moreover, the resulting compounds proved difficult to isolate from the resulting reaction side-products. In 2007, a simplified synthetic procedure was described by Aureggi et al. for the synthesis of 5-substituted tetrazoles using click chemistry. (V. Aureggi et al., Angew. Chem. Int. Ed., 46, 8440-44 (2007)). By reacting organoaluminum azides with a variety of functionalized nitriles, a range of molecules containing a pendent tetrazole were described.
The weakly acidic 1H-tetrazole is an understudied member of the azole family with respect to materials applications. The majority of research on this four-nitrogen, one-carbon azole has been on its pharmacological applications as a carboxylic acid congener. To date, 1H-tetrazole-bearing polymers appear to be unexplored for ion exchange membrane applications, despite prior observation of their potential utility in this area. (N. V. Tsarevski et al., Macromolecules, 37, 9308-9313 (2004)). Tetrazole-bearing polymers have traditionally been polymerized from tetrazole-bearing vinyl monomers or converted post-polymerization from acrylo- or aryl-nitriles in dimethylformamide (DMF) by action of either hydrazoic acid, generated in situ, or azide anion, catalyzed by zinc salts. (V. V. Annenkov et al., J. Polym. Sci., Part A: Polym. Chem., 31, 1903-06 (1993); A. Taden et al., J. Polym. Sci., Part A: Polym. Chem., 40, 4333-43 (2002); V. N. Kizhnyaev et al., Russ. Chem. Rev., 72 (2), 143-164 (2003); P. N. Gaponik et al., Angew. Makromol. Chem., 219, 77-88 (1994); M. R. Huang et al., React. Funct. Polym., 59, 53-61 (2004); N. Du et al., Nat. Mat., 10, 372-75 (2011); N. Du et al., Polymer, 53, 4367-4372 (2012)).